The coiling of armoured cables refers to a storage method where the cable is laid down on a fixed coil pad or in a fixed coil tank, i.e. no rotating turntable is used. The coil pad can for example be a circular or oblong area on the ground. The cable is lowered from above and forms turns around the fixed coil pad or fixed coil tank.
While the cable is laid around the fixed coil pad/tank it is subjected to torsion around its longitudinal axis. For each complete turn around the fixed coil pad/tank the internal twist amounts to 360°. If the cable is unable to accept a 360° twist along each turn around the coil pad it will inevitably form screws, loops or kinks. The twisting hence causes deformations in the cable which can be temporary or permanent. The amplitude and character of the deformations are decisive for the ability of the cable to be coiled. The coiling parameters should be such that all deformations disappear during final installation. The critical issue is the behaviour of the armouring wires under coiling.
FIG. 1 shows the longitudinal cable axis and two cross section planes 3 and 5 of an armoured cable. One armouring wire 1 is visible with an armour angle α measured against the cross sectional plane 3. The armouring wire layer has the diameter DA. During coiling, the two cross sectional planes 3, 5 are rotated by a small angle β with respect to each other. The armouring wire 1 changes its lay angle from α to α′ with α′>α. The armouring wire 1 aligns itself closer to the longitudinal axis A. As a result, the armouring wire 1 occupies a longer length l′ of the cable and the cable therefore has to absorb an additional length Δl=l′−l. The magnitude of Δl depends, among other things, on the induced twist in the cable, the armouring pitch length and the diameter DA of the armouring layer.
The generated additional length of the armouring wire can result in one or a combination of the following: 1) the armouring wires expand from a helix with a first diameter to a larger helix with a second diameter, outwards against the retaining force of the polypropylene yarn of the cable. If the diameter grows too much the yarn can break. 2) If there is little friction between the wires and other cable components the wires can start to migrate longitudinally in the cable. Compression forces in the wires are accumulated until there is an inhomogeneity in the force balance between wires and polypropylene yarn. The yarn can break which results in the formation of a radial protrusion of the armour wires creating a so-called bird cage. Alternatively, if the yarn is intact and the compressive stress in the armour wires becomes too large it can result in Euler buckling and the wires form Z-kinks. 3) The cable experiences a de facto elongation against the retaining force of the conductor.
In the best case the twisting obtained from coiling is equally distributed along the cable. However, if the mechanical properties of the cable are not uniform weak points such as those containing a discontinuity in the armour wires, such as a weld or repair, are more sensitive and may therefore become mechanically overstressed. A welding sleeve is used to repair the amour wire or for jointing the armour layers when jointing two cable lengths. The sleeve is a rigid steel ring onto which the armour wires from each length are welded. The ring prevents the wires from moving radially thereby increasing the stress in the area around the weld when the cable undergoes twisting.
To reduce the risk of deformation, the diameter of the coiled cable has to be increased along a section containing a welding sleeve, e.g. for repairing armouring wire of a cable or for jointing two cables. This complicates the coiling operation, increasing the time, and also the risk of damage. The situation becomes even more complex if coiling or semi-coiling is performed onto a vessel with a limited amount of available space.